1. Field of the Invention This invention relates to apparatus for the bio-decontamination of enclosures and in particular small enclosures.
2. Present State of the Art
U.S. Pat. No. 5,229,071 discloses a batch method and apparatus for controlled release of gaseous air contaminants into the atmosphere through catalytic oxidation while minimizing both the energy required and the volume of waste gas exhausted into the atmosphere. The device has a recirculating gas stream driven by a recirculation fan which moves gas, normally and naturally present at start-up, through a bed of granular catalyst, in an oxidizer and into contact with the surface of a process-gas heater and back to the recirculation fan. The gaseous contaminants may be drawn into this system using a vacuum pump.
U.S. Pat. No. 5,160,700 discloses a sterilizing system including a sealed container for holding a gaseous sterilant under pressure and a first enclosure made at least partially of a gas-permeable material. The container and the articles to be sterilized are disposed in and sealed within the first enclosure, and the container, while in the sealed first enclosure, is manipulated to release gaseous sterilant into the sealed first enclosure. A second enclosure in which the first enclosure is disposed is constructed such that the sterilant released into the first enclosure from the container diffuses through the gas-permeable material of the first enclosure into the second enclosure at a rate capable of establishing sterilizing conditions in the first enclosure during a sterilizing cycle to thereby effect sterilization of the articles in the first enclosure. A moisture-releasing humidifying device is disposed within the first enclosure for releasing moisture into the first enclosure during the sterilization cycle and a regulating system comprising an exhaust device is operable to exhaust the sterilant gas from the second enclosure to minimize the amount of sterilant gas in the second enclosure, thereby providing for minimized residue sterilant in the surrounding work area.
US-A-2003/0086820 discloses that a surface which carries a material which is infected with prions is cleaned with an alkaline cleaning solution to remove as much proteinaceous material as possible from, the surface. The solution contains an alkaline cleaning agent which attacks prions remaining on the surface and which also attacks prions removed from the surface during the cleaning step. After the cleaning step, the surface is exposed to a strong gaseous oxidant, preferably hydrogen peroxide vapor. The hydrogen peroxide or other strong oxidant attacks the prions, particularly the unclumped prion strands, deactivating the prions.
U.S. Pat. No. 3,503,703 discloses a sterilizing apparatus having a gas impermeable barrier and a flexible, collapsible gas impermeable bag having an aperture for receiving articles to be sterilized adapted to be mounted in gastight connection with the barrier. The bag is connected to the barrier in a gastight relationship and exhaust means are provided for reducing the internal pressure in the bag and for circulating air in the bag and valving and controls are provided for carrying out a sterilizing cycle in the bag.
Small enclosures are typically up to about 2 m3 in volume, and include but are not limited to Class II Microbiological Safety Cabinets (MSC). Our International Patent Application PCT/GB03/001386 discloses methods of bio-decontaminating larger enclosures such as rooms or chambers by placing an apparatus to generate the fumigant gas inside the chamber. The technique described works well for rooms and large chambers of a simple nature but is not specifically intended to deal with the problems associated with Class II microbiological safety cabinets and similar enclosures.
The standard technique for bio-decontaminating a Class II MSC is to boil formalin to generate formaldehyde vapour. For this method to be effective substantial amounts of formalin have to be evaporated, the European Standard EN BS 12469 requires 60 ml of formalin plus 60 ml of water to be evaporated for each cubic meter of enclosure volume. Other authorities use smaller amounts of liquid but all of the methods used generate considerable amounts of condensation within the MSC and also form deposits of paraformaldehyde.
Formalin gassing of an MSC has a number of disadvantages; firstly it leaves a residue of formalin and paraformaldehyde that can only be removed by long periods of aeration; secondly the bio-decontamination process is slow, the normal exposure time being eight hours; thirdly it is difficult to ensure that the gas has reached all parts of the MSC especially in the filter plenum, fourthly the vapour is toxic with an Occupational Exposure Limit of 1 ppm, and lastly special precautions have to be taken to avoid leakage of the gas from the MSC, and in some installations the laboratories have to be evacuated during the fumigation process. An alternative to formalin fumigation that overcomes these problems would be of considerable value to laboratory personnel, and one choice of fumigant is hydrogen peroxide vapour providing that it can be deployed in a way which is safe for the user, since it is residue free, is effective and is fast acting.
It may be expected that some of the same difficulties that are encountered with formalin will also be encountered when using hydrogen peroxide as a fumigant. Most, if not all, MSCs leak to some extent. Introducing gas inside a chamber is accompanied by a rise in temperature which causes an increase in internal pressure. This rise in pressure, unless it is controlled, leads to leakage of the fumigant gas to the outside giving rise to a potential hazard to laboratory staff. Hydrogen peroxide and formaldehyde have similar diffusion constants and so it may be expected that the rate at which these two gases would diffuse around the enclosure would be similar. In an MSC it may be expected that bio-decontamination of the plenum chamber using hydrogen peroxide vapour may take some considerable time unless techniques are used to cause the gas to travel into the plenum.
The main advantages of using hydrogen peroxide as the fumigant gas are the facts that it does not leave a residue and that once an adequate gas concentration has been reached the process is very fast. Many, if not most, Class II MSCs that are in use recirculate their exhaust air back to the laboratory, and hence a method is required to remove the hydrogen peroxide vapour at the end of the bio-decontamination cycle.